Valves, in particular for oil hydraulic systems, such as pressure regulating valves for controlling a pressure of a medium flowing through the valve are sufficiently well known. U.S. Pat. No. 4,316,599, for example, describes a proportional pressure regulating valve, which supplies an essentially constant output pressure with a variable input pressure. The output pressure of the pressure regulating valve to be controlled is preselected by a current signal originating from a corresponding control electronics for an actuating magnet of a magnetic system. Proportional pressure regulating valves in this regard are preferably designed as directly controlled piston slide valves in a three-way embodiment. Valves in this regard are used in oil hydraulic systems, for example, for controlling clutches in manual transmissions or automatic transmissions for a targeted influence on the pressure buildup and dissipation of pressure on hydraulic consumers, for remote setting of the pressure or for pilot control of hydraulic valves and hydraulic actuators in a variety of designs.
DE 10 2005 006 321 A1 discloses a valve, in particular a proportional pressure limiting valve, having a valve housing with at least two fluid connections, wherein the valve can be connected to a hydraulic drive system at a predefinable load pressure. The valve is also suitable for use in hydraulic systems with fluid media of a low viscosity due to the fact that a valve piston, which is operatively connected to a pilot seat by an energy accumulator, can be controlled by a control device. The applied consumer pressure on a fluid connection acts at least on the valve piston such that a fluid flow between the two other fluid connections in both directions within the valve housing is possible, depending on the applied consumer pressure and the actuating force of the control device so far. A main stage of the valve can be controlled by a pilot stage in the direction opposite the action of a return device by an energizable actuating device. The valve also has a pressure relief stage in its pilot stage.
DE 10 2007 015 077 A1 discloses a slide valve for hydraulic systems such as proportional pressure limiting valves or pressure compensators, having a valve housing preferably designed in the form of a screw-in cartridge. A valve slide is axially displaceable and has an inflow opening through which fluid flows into the valve housing. At least one outflow opening is formed in the wall of the valve housing and can be closed by the valve slide, depending on the axial position of the valve slide, or can be released for the outflow of fluid out of the valve housing. To reduce cavitation phenomena with the slide valve, an annular groove is cut into the inside wall of the valve housing, with the annular groove being positioned with respect to its axial position at the height of the at least one outflow opening. A control edge of the valve slide then overruns the annular groove in its movement out of the position that closes the outflow opening and into the release position. In some applications of the hydraulic or pneumatic system, a fail-safe behavior of valves is desired in this regard. In the event of a malfunction of the control of a valve, in particular in a failure of an energizable actuating device for the valve, the valve assumes a predefinable function. In particular a valve element of the valve assumes a predefinable switch position. The functions or operating points of the hydraulic system that are desired in a hydraulic system can then be selected. One example is hydraulic or pneumatic switch elements in cooling systems because when their control fails, the cooling system in this regard nevertheless maintains its function.
DE 44 42 085 A1, EP 0 030 083 A2, U.S. Pat. No. 4,750,704 and DE 30 42 015 A1 each have pilot-controlled proportional pressure regulating valves having a pressure relief stage.